1. Field of the Invention
The present invention relates to a CMOS image sensor and a method for fabricating the same, and more particularly, to a CMOS image sensor and a method for fabricating the same, to improve reliability of a transistor for a driving part, and to improve a voltage of a photodiode.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor can be broadly categorized into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor.
The charge coupled device (CCD) includes a plurality of photodiodes (PD) aligned in a matrix-type configuration and converting light signals into electric signals, a plurality of vertical charge coupled devices (VCCD) formed between each vertical photodiode aligned in a matrix-type configuration and vertically transmitting electric charges generated from each photodiode, a horizontal charge coupled device (HCCD) horizontally transmitting the electric charges transmitted by each of the vertical charge coupled devices (VCCD), and a sense amplifier sensing and outputting the horizontally transmitted electric charges.
However, the charge coupled device (CCD) has disadvantages of a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photo process. In the charge coupled device (CCD), a control circuit, a signal processing circuit, an analog to digital (A/D) converter circuit, and so on cannot be easily integrated into a charge coupled device chip, thereby having the problem of forming compact-size products.
Recently, the complementary metal oxide semiconductor (CMOS) image sensor has been considered to be the next generation image sensor that can resolve the problems and disadvantages of the charge coupled device (CCD). The CMOS image sensor is a device adopting a CMOS technology using the control circuit, the signal processing circuit, and so on as a peripheral circuit, so as to form MOS transistors corresponding to the number of unit pixels on a semiconductor substrate, in order to sequentially detect the electric signals of each unit pixel by using a switching method, thereby representing an image.
Since the CMOS image sensor uses a CMOS fabrication technology, the CMOS image sensor is advantageous in that it has low power consumption and has a simple fabrication method through less photo process steps. In the CMOS image sensor, a control circuit, a signal processing circuit, an A/D converter circuit, and so on can be integrated in a CMOS image sensor chip, thereby enabling the product to be fabricated in a compact size. Accordingly, the CMOS image sensor is currently being extensively used in various applied technologies, such as digital still cameras and digital video cameras.
Meanwhile, the CMOS image sensor is categorized into 3T-type, 4T-type, and 5T-type according to the number of transistors, wherein the 3T-type CMOS image sensor is comprised of one photodiode and three transistors, and the 4T-type CMOS image sensor is comprised of one photodiode and four transistors. A layout of a unit pixel in the 3T-type and 4T-type CMOS image sensors will be described as follows.
FIG. 1 is a layout of a unit pixel in a 3T-type CMOS image sensor. FIG. 2 is a layout of a unit pixel in a 4T-type CMOS image sensor. FIG. 3 is an equivalent circuit diagram of a unit pixel in a general 3T-type image sensor. FIG. 4 is an equivalent circuit diagram of a unit pixel in a general 4T-type image sensor.
In a unit pixel of a general 3T-type image sensor, as shown in FIG. 1, an active area 10 is defined. Then, one photodiode 20 is formed in a large sized portion of the active area 10, and respective gate electrodes 120, 130, and 140 of three transistors are overlapped with the remaining portion of the active area 10. That is, a reset transistor Rx is formed by the gate electrode 120, a drive transistor Dx is formed by the gate electrode 130, and a select transistor Sx is formed by the gate electrode 140. In this case, impurity ions are implanted to the active area 10 of the respective transistors except portions below the gate electrodes 120, 130, and 140, thereby forming source/drain regions in the respective transistors. Accordingly, a power voltage Vdd is applied to the source/drain regions between the reset transistor Rx and the drive transistor Dx, and a power voltage Vss is applied to the source/drain regions at one side of the select transistor Sx.
Also, as shown in FIG. 2, in a unit pixel of a general 4T-type image sensor, an active area 10 is defined. Then, one photodiode 20 is formed in a large sized portion of the active area 10, and respective gate electrodes 110, 120, 130, and 140 of four transistors are overlapped with the remaining portion of the active area 10. That is, a transfer transistor Tx is formed by the gate electrode 110, a reset transistor Rx is formed by the gate electrode 120, a drive transistor Dx is formed by the gate electrode 130, and a select transistor Sx is formed by the gate electrode 140. At this time, impurity ions are implanted to the active area 10 of the respective transistors except portions below the gate electrodes 110, 120, 130, and 140, thereby forming source/drain regions in the respective transistors. Accordingly, a power voltage Vdd is applied to the source/drain regions between the reset transistor Rx and the drive transistor Dx, and a power voltage Vss is applied to the source/drain regions at one side of the select transistor Sx.
That is, as shown in FIG. 1 to FIG. 4, in case of both 3T-type and 4T-type image sensors, a contact region is formed in the active area between the reset transistor Rx and the drive transistor Dx, whereby the power voltage Vdd is applied to the photodiode, and the power voltage Vss is applied to the select transistor Sx.
Also, the transistors comprising of the 3T-type and 4T-type image sensors are divided into a photodiode transistor and a driving part transistor according to their functional characteristics. In this state, the photodiode transistor functions as a switching transistor for inputting/outputting data to/from the photodiode. For example, in case of the 3T-type image sensor, the reset transistor is corresponding to the photodiode transistor of the switching transistor. In case of the 4T-type image sensor, the reset transistor and the transfer transistor are corresponding to the photodiode transistor of the switching transistor. Also, the driving part transistor functions as a transistor for outputting data (optical electric charge) outputted from the photodiode through a column line to the external. For example, in case of the 3T-type and 4T-type image sensors, the drive transistor and the select transistor function as the driving part transistor. When dividing the photodiode transistor and the driving part transistor in the unit pixel, the photodiode transistor is positioned between the photodiode and the power voltage Vdd, and the driving part transistor is positioned between the power voltage Vdd and Vss.
A photodiode transistor and a driving part transistor of a related art CMOS image sensor will be described as follows.
FIG. 5 is a cross sectional view of the related art CMOS image sensor along I–I′ of FIG. 5. As shown in FIG. 5, a p-type epitaxial layer 101 is formed on a p-type semiconductor substrate 100, to form a large and deep depletion region in a photodiode of the p-type semiconductor substrate 100. Then, after defining an active area (‘10’ of FIG. 1 or FIG. 2), a gate insulating layer 11 and respective gate electrodes 120, 130, and 140 of a reset transistor Rx, a drive transistor Dx, and a select transistor Sx are formed on the p-type epitaxial layer 101. At this time, the gate insulating layer 11 corresponding to the respective gate electrodes 120, 130, and 140 of the reset transistor Rx, the drive transistor Dx, and the select transistor Sx has the same thickness. After that, source/drain regions 10a, 10b, 10c, and 10d are formed in the active area (‘10’ of FIG. 1 or FIG. 2) between each gate electrode 120, 130, and 140.
An operation of the related art CMOS image sensor will be described as follows.
First, the power voltage Vdd is applied to the photodiode 20 through the photodiode transistor (reset transistor or transfer transistor). After a predetermined time period, the optical electric charge generated in the photodiode 20 is outputted through the photodiode transistor. In this case, to generate a great amount of optical electric charges in the photodiode, it is required to obtain a high voltage applied to the photodiode through the photodiode transistor.
The voltage applied to the photodiode is a threshold voltage (Vdd−Vth) of the power voltage Vdd-photodiode transistor Vth.
That is, to make the high voltage of the photodiode, it is required to obtain the high power voltage Vdd, and to lower the threshold voltage. Furthermore, in order to lower the threshold voltage of the reset transistor or the transfer transistor, it is necessary to decrease the thickness of the gate insulating layer provided in each transistor.
However, in case of lowering the threshold voltage of the transistor, it may have the problem of reliability in the driving part transistor of the drive transistor and the select transistor. The driving part transistor is positioned between the Vdd and Vss terminals, whereby a great amount of positive electric charges flow in the driving part transistor as compared with that in the photodiode transistor. As a result, as the power voltage Vdd becomes high, the reliability of the driving part transistor is deteriorated.
The thickness of the gate insulating layer is correlated with the reliability of the driving part transistor, whereby the operation reliability of the CMOS image sensor is very sensitive to the thickness control of the gate insulating layer.
Accordingly, in the related art CMOS image sensor, the gate insulating layers corresponding to the photodiode transistor and the driving part transistor have the same thickness. To improve the output voltage of the photodiode, it is necessary to decrease the thickness of the gate insulating layer comprising the photodiode transistor, as well as to apply the high power voltage Vdd. In this case, if decreasing the thickness of the gate insulating layer, it has a bad influence to the reliability of the driving part transistor.